1. Field of the Invention
This invention relates to Nuclear Magnetic Resonance (N.M.R.) probe systems of the so called crossed-coil type wherein a transmitter coil and a receiver coil are arranged with their magnetic axes substantially orthogonal to each other so as to minimize electromagnetic coupling therebetween. The invention is particularly related to the substantial cancellation of any spurious RF voltage set up in the receiver coil by undesired electromagnetic and electrostatic couplings between the receiver section of the probe (receiver coil and appertaining RF circuit) and the transmitter section (transmitter coil and appertaining RF circuit).
It must be understood that for the purposes of the present specification the phrase "probe system" refers either to the probe along, i.e., a plurality of functionally interrelated electrical and mechanical parts within a housing adapted to be positioned in predetermined attitude within the polarizing magnetic field of an NMR spectrometer, or to the probe and any ancillary devices required for its control, wherever said devices may be mounted, e.g., on the probe externally thereof or at some remote location such as on the spectrometer control panel or an accessory panel.
2. Description of the Prior Art
An ideal crossed-coil probe system would ensure that in operation, no RF voltage appeared in the receiver coil other than in correspondence of those regions of the scanned NMR spectrum where resonances actually occurred in the sample under analysis. In other words, the receiver coil would respond to nuclear induction only. As a first step towards approximating this desirable performance, it is usual to take great care over the electromagnetic orthogonality of the transmitter and receiver coils, but even if perfection could ever be achieved, stray electromagnetic couplings between transmitter and receiver sections would cause a standing RF voltage to be induced in the receiver coil.
It is clear than any electrostatic coupling must produce a similar result and in some probe designs, the receiver coil is in fact surrounded by a Faraday screen. Again, electrostatic screening cannot be perfect.
We must conclude that in a probe system of the type referred to, stray electromagnetic and electrostatic couplings are unavoidable and it follows that the presence of spurious RF voltage in the receiver coil must be expected. Cancellation by some suitable means is the practical approach to the problem. The cancelling operation is known in the art as "probe balancing".
Various probe balancing schemes have been suggested in the past and one that has been in use for a long time relies on trimming devices in the form of guides and paddles which distort the electromagnetic flux from the transmitter coil and deliberately introduce an induction vector of the right amplitude and phase to set up an RF voltage in the receiver coil which substantially cancels the spruious RF voltage therein.
A much improved scheme is disclosed in U.S. Pat. No. 3,603,871, wherein the transmitter coil is arranged in two halves and the electromagnetic orthogonality of each with respect to the receiver coil is deliberately imparied by orienting them (or shaping them) in such manner as to give rise at the receiver coil to electromagnetic induction vectors of opposite signs and similar amplitudes. By repeated phase and amplitude adjustments of the RF voltage across each half in turn, a resultant vector is produced which is the sum of the standing vector initially existing (as a result of stray couplings) and the vector produced by one of the transmitter coil halves. The resultant vector is cancelled by an equal and opposite induction vector produced by the other coil half. This scheme has been found quite satisfactory in practice and one of its virtues is the total elimination of the Faraday screen since it can be made to have enough cancelling range to cope with any standing RF voltage no matter whether its origin is electromagnetic or electrostatic.
We have now recognized that probe balance methods which resort to steering of the electromagnetic flux from the transmitter coil cause undesirable magnetic and electromagnetic disturbances in the critical magnetic volume of high intrinsic homogeneity in which the NMR sample is located while supported in the probe.
Whether flux steering is implemented by interposing flux guides between the transmitter coil and the receiver coil or by interfering with the orthogonality of the transmitter coil with respect to the receiver coil, the irradiation pattern of the transmitter coil is clearly distorted as a result. This has an adverse effect on the sharpness of the NMR response in spin-decoupling experiments. In addition, the magnetic homoengeity of the magnetic volume "seen" by the NMR sample tends to be degraded. The reason for this concomitant effect is not immediately apparent and is worth explaining.
Practical materials used in the construction of probe elements that in operation will be located within the field of the polarizing magnet near the working gap thereof (among them the copper wire of the transmitter coil) have a magnetic susceptibility that is almost inevitably other than zero. They will therefore tend to increase the density of the lines of force passing therethrough compared with what their density would be in free air (which is of zero susceptibility) if they are paramagnetic (susceptibility greater than zero) and to decrease it if they are diamagnetic (susceptibility smaller than zero). In either case, their presence will obviously have some de-homogenizing effect on the magnetic field in the working gap, since it interferes with the honogeneous distribution of the magnetic lines of force thereat. In order that this effect be minimized, the materials are arranged to give reasonable uniformity of susceptibility around the working-gap centre. The introduction of flux guides, which by their very nature, are devices of irregular susceptibility, is bound to disturb said uniformity quite significantly. Interfering with the shape and orientation of the transmitter coil to provide a small controllable electromagnetic coupling between the transmitter coil and the receiver coil has a similar but much less significant effect.
Another problem associated with flux steering as referred to is that it tends to restrict the cancellation range since a wider range can only be achieved at the cost of increased megnetic and electromagnetic distortion.
An attempt to avoid flux steering methods in achieving NMR probe balance has been made by resorting to means remote from the carefully controlled operational environment of the probe within the NMR magnet for generating an RF voltage matching in frequency, phase and amplitude the spurious RF voltage in the receiver coil. The solution involves unnecessary complexity of apparatus and is operationally inconvenient in that the cancelling action is subject to drift mainly because it is generated outside the probe environment.
The problem left essentially unresolved by the prior art is how to provide a crossed-coil NMR probe system including probe balancing means which:
a. avoid flux steering and thus have no significant effect on the magnetic homogeneity of the polarizing field of the NMR spectrometer and the uniformity of the sample irradiating pattern produced by the transmitter coil; PA1 b. provide an ample cancelling range to take care without compromise of the contributions to the spurious RF voltage due to electrostatic induction effects, without requiring the use of a Faraday screen; PA1 c. are within the immediate environment of the volume occupied in operation by the NMR sample, so as to enable inherently drift-free balancing operation; PA1 d. allow considerable latitude in design.